Investigation of rheology behavior of solder paste

Abstract

This paper presents results of investigation of rheology behavior of three solder pastes with various densities (approaching viscosities). Comparison of three viscosity structural models and one thixotropy model were used for description of rheological flow curve of these solder pastes samples. Solder pastes are metal alloys suspended in flux medium and spotlight of this paper is on modeling of viscosity structural models of solder paste. Solder paste rheology is the most important parameter which affects the paste printing process, the reflow soldering performance, and wherefore the resulting solders joint quality and reliability. The rheology behavior can be correlated to the performance of the materials during the stencil printing (dispensing) process. Solder pastes exhibit non-Newtonian rheological characteristics such as shear thinning i.e. the reduction of viscosity with increased shear in dependency on temperature and time. Thixotropy and viscosity structural models help to understand the rate of structural change of solder paste. The viscosity values from structural models are compared with empiric measured values.